Fibre reinforced polymer composites thermoplastic matrices properties

To maximise the mechanical properties of short fibre-reinforced thermoplastic composites, it is necessary to achieve effective stress transfer between fibre and polymer matrix and appropriate fibre alignment in the finished component relative to the direction of applied stress. The former requirement is governed primarily by the method of compounding, whilst fibre orientation is very dependent on moulding conditions [76]. 

So far when considering carbon fibre composites the matrix has been a thermosetting resin or polymer. The relative merits of thermosetting and thermoplastic polymer matrices were discussed briefly in Chapter 3. Several thermoplastic matrices are compounded with short carbon or glass fibres. In Table 5.22 however the properties of unidirectional, continuous, carbon fibre-reinforced materials are illustrated. 

Most biodegradable composites being tested employ a thermoplastic matrix. There are inherent difficulties in interfacial wetting between linear thermoplastics and reinforcing fibres which limit optimum stress transfer between the fibre and matrix. This also deleteriously affects hydrolytic stability [63]. While the use of solid polymer without fibre has been considered [24,71-73], problems such as premature cracking can occur. Short fibre composites have been made and tested [63], but properties are not optimal. 

The mechanical properties of plastics materials may often be considerably enhanced by embedding fibrous materials in the polymer matrix. Whilst such techniques have been applied to thermoplastics the greatest developents have taken place with the thermosetting plastics. The most common reinforcing materials are glass and cotton fibres but many other materials ranging from paper to carbon fibre are used. The fibres normally have moduli of elasticity substantially greater than shown by the resin so that under tensile stress much of the load is borne by the fibre. The modulus of the composite is intermediate to that of the fibre and that of the resin. 

Composites are made up of individual materials these are referred to as constituent materials. The purpose of a composite is to create a material that combines its constituent parts in some beneficial way. The two main categories of constituent materials are the matrix and the reinforcement. The synthetic matrix materials are either thermoplastic or thermosetting resins. These polymers bind the reinforcement together and determine the physical in-service properties of the composite material. Polymers can also act as reinforcing material in composites Kevlar for instance, is a synthetic polymer fibre that is very strong and imparts toughness to a composite. 

The problem of reinforcing PVC with fibres differs from that of polyolefins. This is because PVC needs another additive such as plasticiser, which should be added. The improvement in dispersion and adhesion between the wood particles and the polymer matrix showed that the mechanical strengths can be greatly enhanced, while the rheology or processing of thermoplastics/wood flour composites is seldom studied [34,35]. Recently, the effect of low levels of plasticiser on the rheology and mechanical properties of PVC/newsprint fibre composites has been reported [36]. 

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